A glass casting process produces glass objects by melting glass in a mold of a desired shape, allowing the glass to cool in the mold until it is sufficiently hard, and then separating the glass from the mold. There are various techniques for creating molds for glass objects, each technique with its own advantages.
Rectangles may be cast by creating containments using firebrick or ceramic. For example, kiln shelves may be cut in long strips which may be arranged as a rectangle mold. In some instances, the containments may be lined with fiber paper or other refractory. One drawback of creating containments out of firebrick or ceramic is a limited number of shaping options. In another example, a vermiculite board may be used for making straight sided casting boxes. However, vermiculite may be quite fragile and hard to create complex shaped containments, especially after the vermiculite undergoes a burnout process where a containment is fired before use to remove the binders.
In another example containment, fiber-lined rings may be used to cast glass. For example, a cut section of pipe may be used to create a containment. Unfortunately, pipes come in a limited range of sizes and many pipes have sidewalls that are too thick to allow their shape to be easily changed. Pipes may be made out of material that does not expand and contract in a linear fashion during firing, for example a carbon steel pipe, and therefore may be problematic for glass casting processes. Additionally, many pipe materials have limited durability, may corrode rapidly at a high temperature and therefore may require cutting multiple sections of pipe.
Existing glass casting approaches often utilize metal casting techniques such as sand casting, lost wax, etc. In a sand casting example, a positive model is made, for example from wood stock. Then, the positive model is pressed into a prepared sand bed, making an impression that is then filled with glass or metal. In some instances, a three-dimensional casting may be created by placing a top mold inverted above a bottom mold. There are two main variants (green sand, and sodium silicate).
In a green sand approach, sand is mixed with a binder to hold the sand in a particular shape. For example, in a metal casting sand may be mixed with motor oil. Casting is usually done with molten glass from a furnace, with carbon black used as a mold release. However, using this approach in a kiln is inefficient as the sand is also heated. Furthermore, since using this approach in a kiln requires more heat, it may take a relatively long time and also may result in overcooking the glass that is being cast. Additionally, a sand based approach may overcook glass that is being cast, causing an unintended color shift in the glass and potentially causing incompatibilities between different portions of a casting.
Additionally, sodium silicate may be mixed with sand & packed around a model, wherein the sand is then flooded with carbon dioxide causing the sand to lock together into a rigid mold. This approach can provide fine detail when used with sand of a particular grain size and within a certain sand to binder ratio. Unfortunately, this approach typically requires creating a new mold for each casting process.
Lost wax is another traditional casting method. In this approach, a wax model may be made and surrounded with a castable refractory. When the refractory is dry, the mold may then be heated to remove the wax, and molten glass is introduced through channels called sprues to fill the voids. While this approach may be automated, it also requires creating a new mold for each new casting process.
In a plaster investment casting approach, a plaster/silica mix or castable refractory may be used. This approach involves suspending a model shape in a container and pouring the castable refractory in around the model shape. Once the castable refractory sets-up and dries, the model may be burned out to create a ready to use mold. A plaster investment casting approach also may generate uneven cooling of a glass casting, which may cause the casting to fail or cause an unintended change in geometry or may result in uneven quality of the glass casting.
In a dip and sprinkle casting approach, a wax model is dipped in a binder solution and sprinkled with refractory grit. The model is then repeatedly dipped and sprinkled with successively coarser grits until a shell is built up around the model. After the model is removed, glass may be placed into the mold created by the shell. In this approach, after casting the mold may then be removed by sandblasting or other destructive method. A variant of this may be used for art glass where a plaster mix is first combined with loose refractory fiber, then hand-packed onto a model in layers.
The approaches described above are traditional metal casting methods that may be used in a glass casting process. These methods typically require a separate process for making a model, which is then used to make a mold. Further, some part of each of these approaches is destroyed in the process. Additionally, some of these traditional metal casting methods use a significant amount of refractory, which requires more energy to be heated to a casting temperature and also acts as an insulator to impede heating of the cast material. The following paragraphs describe approaches specifically developed for glass casting.
In a cut fiber investment glass casting approach, multiple designs are cut into sheets of fiber paper, and the sheets of fiber paper are then stacked to achieve a three dimensional shape. For example, a cut fiber investment technique is sometimes used for open-faced castings. Although this approach allows a range of casting shapes, the shapes must be cut into multiple sheets of paper, thus resulting in a laborious process. Further, as fiber paper is relatively expensive and a cut fiber investment approach relatively labor intensive, this approach may be cost prohibitive.
Another approach specifically designed for glass casting is a rigidized fiber mold approach. In this approach refractory fiber paper, or a blanket or a board, is soaked with a rigidizing agent such as sodium silicate, and then shaped over a form. After the fiber paper, blanket or board dries, it is rigid enough to be used as a mold or a slumping form.
In a Pate de Verre casting approach, a plaster mold may be cast around a model, for example a clay or wax model, then the model is removed. Next, glass frit or powder is mixed with a binder and packed in the mold and then fired one or more times. Unfortunately, in this approach the mold is typically only able to produce one piece.
In a freeze and fuse glass casting approach, glass frit and water is made into a paste, packed into a flexible mold, and then frozen. Next, the frozen paste is placed in a kiln and the glass is heated up to tack the pieces together, sometimes called a tack fuse.
These approaches may require creating molds for every glass casting and consuming essential materials during the process. Further, these approaches may be costly and may provide an inefficient use of materials, energy, time, and labor.